Open the CD/DVD tray
I was confused after starting OS 9 for the first time on my iMac G4. Although, there was a key for opening the bay on the keyboard, the tray wouldn't open.
The trick is to use an application on the installer disk in the /CD Extras folder (or download it from Macintosh Garden).
So, press the mouse button during booting until the tray opens or boot into OS X to insert the disk. Copy the folder /CD Extras/Eject Extras on your hard disk. I copied it into the /Applications (Mac OS 9)/Utilities folder.
To make things more convenient, I want to use a function key to open the tray. Press F13 (for example) and you will be asked if you want to open the Keyboard control panel. Do so and assign the Eject application to your favorite function key.
Network
Next up is establishing a network connection. There is a control panel called TCP/IP (apple menu -> control panels) where the necessary settings can be entered. For some reasons DHCP is not working properly all the time, however manually entering the settings seems to work fine.
Access to NAS
Launch the network browser (apple menu):
Select Connect To Server… and enter the ip address of the NAS.
Printing
I have not yet found a way to print from OS 9 to my printer connected to my main OS X machine.
So, the next best thing seems to install Acrobat 5 on OS 9, print into a PDF and save that to my NAS to print it from there on OS X.
Don't forget to select Adobe as your printer in Chooser (apple menu).
VNC
An excellent way to control your OS 9 machine from any other computer. I installed OS9 VNC 3.5.0. You need to copy the VNC extension to the /System Folder/Extensions folder and restart the machine.
Changing resolution and colors
It is most annoying when applications change the desktop resolution and/or color depth on their own. Once you quit that application, your icons and windows on the desktop are all over the place. Desktop Resetter eases the pain. You can save different desktop layouts and restore them later.
Mounting volumes
Most old software you can find on the internet comes as (some kind of) mountable volume. I started with burning them all to CDs under OS X and then install the applications under OS 9. But there is a better way, once you connect your OS 9 to your local network. Download the image to OS 9 and mount it there with Toast Titanium 5. This does not work all the time (since some applications demand a physical CD in the drive), but often enough to save a lot of CDs.
Formatting floppy discs
I bought an external USB floppy disc drive. As long as a disc has been formatted before it works fine most of the time. But when working with "brand new" discs that have not been formatted before, I rarely manage to format them with the USB drive. Most of the time the computer freezes. So far I've tried Mac OS 9, MacOS X and even Windows 10 with no success. So, I tried to format the discs with the Power Macintosh 5500 (with an internal floppy disc) and there formatting works just fine. Working on other machines with these formatted floppy discs and the external USB drive is no problem at all.